Shares of Chelsea Therapeutics International Ltd. sank in after-hours trading Tuesday after the company reported mixed results from a clinical trial of its drug Northera, which is intended to help prevent dizziness and falling in patients with Parkinson's disease.

Chelsea Therapeutics said Northera was more effective than a placebo at reducing dizziness and lightheadedness after a week of treatment. However it said the results after more than a week were less clear, as the difference between Northera and placebo was not statistically significant. The company also said the Northera patients fell less frequently and were injured in falls less often, but that difference between Northera patients and the placebo group wasn't statistically significant.

The results came from a late-stage clinical trial of the drug. In July Chelsea Therapeutics said the Food and Drug Administration suggested it run another trial, as the current study was not likely to provide enough evidence to support approval of Northera.

Chelsea Therapeutics does not have any approved products and Northera is the company's most advanced experimental drug.

Chelsea Therapeutics stock lost 43 cents, or 24 percent, to $1.36 in late trading after falling 4 cents, or 2.1 percent, to $1.79 during the regular session. The shares are down 67 percent over the last year.

Northera, or droxidopa, is designed to treat neurogenic orthostatic hypotension, or a drop in blood pressure upon standing. The condition is common in patients with Parkinson's disease, which is the second most common neurodegenerative disorder in the U.S.